1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera.
2. Description of the Prior Art
A zoom lens system for a compact camera does not require a long back focal distance, unlike a zoom lens system for a single lens reflex camera which requires a space, behind the photographing lens system, in order to provide a quick-return mirror. Accordingly, a retrofocus-type lens system having a negative lens group and a positive lens group, in this order from the object, is employed in a single lens reflex camera, whereas a telephoto-type lens system having a positive lens group and a negative lens group, in this order from the object, is generally employed in a compact camera.
In a zoom lens system of a telephoto-type, in order to increase the zoom ratio, a three-lens-group arrangement is employed. More specifically, the positive lens group on the object side is divided into two lens groups so that the zoom lens system has the three lens groups in total, i.e., a positive lens group, the other positive lens group, and the negative lens group, in this order from the object. It is noted that a zoom lens system of this telephoto three-lens-group type is widely used. In such a three-lens-group zoom lens system, by varying the distance between the positive first lens group and the positive second lens group, the zoom ratio of the entire zoom lens system can be increased while mainly field curvature is corrected. Furthermore, since there are more distance parameters than in the case of a zoom lens system of a two-lens-group arrangement, the overall length of the three-lens-group arrangement can be maintained relatively shorter even if the zoom ratio is increased.
However, in such a three-lens-group arrangement, if the zoom ratio is increased while the overall length thereof is maintained shorter at the long focal length extremity, aberration fluctuations due to zooming are made larger, so that the correcting of aberrations over the entire focal length range becomes difficult. In particular, chromatic aberration at the short focal length extremity and the long focal length extremity occur in opposite directions, so that the correcting of chromatic aberration over the entire focal length range becomes difficult.
Furthermore, in a case where a diaphragm is provided between lens elements constituting the positive second lens group in a zoom lens system of a three-lens-group arrangement, the diameter of the front (positive first) lens group can advantageously be reduced. However, since the positive second lens group suffers greatly from deterioration on the image-forming performance due to manufacturing error, higher precision is required therein with respect to the distance and decentration between a front portion of the positive second lens group before the diaphragm, and a rear portion thereof behind the diaphragm. Consequently, the production costs increase, and/or the mechanical structure becomes more complicated, so that the overall size of the zoom lens system increases.